This invention generally relates to a sootblower device for cleaning interior surfaces of large-scale combustion devices. More specifically, this invention relates to new designs of nozzles for a sootblower lance tube providing enhanced cleaning performance.
Sootblowers are used to project a stream of a blowing medium, such as steam, air, or water against heat exchanger surfaces of large-scale combustion devices, such as utility boilers and process recovery boilers. In operation, combustion products cause slag and ash encrustation to build on heat transfer surfaces, degrading thermal performance of the system. Sootblowers are periodically operated to clean the surfaces to restore desired operational characteristics. Generally, sootblowers include a lance tube that is connected to a pressurized source of blowing medium. The sootblowers also include at least one nozzle from which the blowing medium is discharged in a stream or jet. In a retracting sootblower, the lance tube is periodically advanced into and retracted from the interior of the boiler as the blowing medium is discharged from the nozzles. In a stationary sootblower, the lance tube is fixed in position within the boiler but may be periodically rotated while the blowing medium is discharged from the nozzles. In either type, the impact of the discharged blowing medium with the deposits accumulated on the heat exchange surfaces dislodges the deposits. U.S. Patents which generally disclose sootblowers include the following, which are hereby incorporated by reference U.S. Pat. Nos. 3,439,376; 3,585,673; 3,782,336; and 4,422,882.
A typical sootblower lance tube comprises at least two nozzles that are typically diametrically oriented to discharge streams in directions 180° from one another. These nozzles may be directly opposing, i.e. at the same longitudinal position along the lance tube or are longitudinally separated from each other. In the latter case, the nozzle closer to the distal end of the lance tube is typically referred to as the downstream nozzle. The nozzle longitudinally furthest from the distal end is commonly referred to as the upstream nozzle. The nozzles are generally but not always oriented with their central passage perpendicular to and intersecting the longitudinal axis of the lance tube and are positioned near the distal end of the lance tube.
Various cleaning mediums are used in sootblowers. Steam is commonly used. Cleaning of slag and ash encrustations within the internal surfaces of a combustion device occurs through a combination of mechanical and thermal shock caused by the impact of the cleaning medium. In order to maximize this effect, lance tubes and nozzles are designed to produce a coherent stream of cleaning medium having a high peak impact pressure on the surface being cleaned. Nozzle performance is generally quantified by measuring dynamic pressure impacting a surface located at the intersection of the centerline of the nozzle at a given distance from the nozzle. In order to maximize the cleaning effect, it is generally preferred to have the stream of compressible blowing medium fully expanded as it exits the nozzle. Full expansion refers to a condition in which the static pressure of the stream exiting the nozzle approaches that of the ambient pressure within the boiler. The degree of expansion that a jet undergoes as it passes through the nozzle is dependent, in part, on the throat diameter, the length of the expansion zone within the nozzle, and the expansion angle.
Classical supersonic nozzle design theory for compressible fluids such as air or steam require that the nozzle have a minimum flow cross-sectional area often referred to as the throat, followed by an expanding cross-sectional area (expansion zone) which allows the pressure of the fluid to be reduced as it passes through the nozzle and accelerates the flow to velocities higher than the speed of sound. Various nozzle designs have been developed that optimize the expansion of the stream or jet, as it exits the nozzle. Constraining the practical lengths that sootblower nozzles can have is a requirement that the lance assembly must pass through a small opening in the exterior wall of the boiler, called a wall box. For long retracting sootblowers, the lance tubes typically have a diameter on the order of three to five inches. Nozzles for such lance tubes cannot extend a significant distance beyond the exterior cylindrical surface of the lance tube. In applications in which two nozzles are diametrically opposed, severe limitations in extending the length of the nozzles are imposed to avoid direct physical interference between the nozzles or an unacceptable restriction of fluid flow into the nozzle inlets occurs.
In an effort to permit longer sootblower nozzles, nozzles of sootblower lance tubes are frequently longitudinally displaced. Although this configuration generally enhances performance, it has been found that the upstream nozzle exhibits significantly better performance than the downstream nozzle. Thus, an undesirable difference in cleaning effect results between the nozzles.